1. Field of the Invention
The present invention relates to a phosphor in which luminescence generated by applying electric field (electro luminescence) and to a light emitting device using the phosphor. In particular, the present invention relates to a light emitting device in which an organic compound is used for a phosphor. Fluorescence and phosphorescence are included in the electro luminescence. The present invention relates to a light emitting device to which light emission by one or both of the fluorescence and phosphorescence is applied.
2. Description of the Related Art
A back light or a front light is used as the typical form of a display device using liquid crystal, and the structure is such that an image is displayed by means of the light. The liquid crystal display device is adopted as an image displaying means in various electronic devices, but has a defect from the viewpoint of its structure that a viewing angle is narrow. On the contrary, a display device using a phosphor in which electro luminescence is obtained has a wide viewing angle and is excellent in visibility. Thus, the display device using a phosphor has attracted attention as the display device in the next generation.
A light emitting element in which an organic compound is used for a phosphor (hereinafter referred to as organic light emitting element) is structured by appropriately combining a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer, which are formed by the organic compound, between a cathode and an anode. Here, the hole injecting layer and the hole transporting layer are separately shown, but these are the same in a meaning that a hole transporting property (hole mobility) is particularly an important characteristic. In order to distinguish these layers for convenience, the hole injecting layer indicates the layer that contacts the anode, and the layer that contacts the light emitting layer is referred to as the hole transporting layer. Further, the layer that contacts the cathode is referred to as the electron injecting layer, and the layer that contacts the light emitting layer is referred to as the electron transporting layer. The light emitting layer may also serve as the electron transporting layer, and thus, is also referred to as a light emitting electron transporting layer. The light emitting element formed by combining these layers shows a rectification characteristic, and has the same structure as a diode.
The light emitting mechanism is considered such that an electron injected from the cathode and a hole injected from the anode are recombined in a layer comprised of a phosphor (light emitting layer) to form an exciton, and the exciton emits light when returning to a base state. There are emission of light from a singlet excitation state (fluorescence) and emission of light from a triplet excitation state (phosphorescence). Luminance reaches several thousands to several tens of thousands cd/m2. Thus, it is considered that the light emission mechanism can be applied to a display device and the like in principle. However, various types of deterioration phenomena exist, and remain as a problem that impedes putting of the display device and the like to practical use.
The deterioration of the phosphor comprised of the organic compound or the organic light emitting element is considered to arise from the five factors mentioned below. The factors are (1) chemical deterioration of the organic compound (through the excitation state), (2) melting of the organic compound due to heat generation at the time of drive, (3) dielectric breakdown caused by a macro defect, (4) deterioration of an electrode or an interface of the electrode and an organic layer and (5) deterioration that arises from instability in an amorphous structure of the organic compound.
The above factors (1) to (3) are caused by driving the organic light emitting element. Heat is inevitably generated by such that a current in the element is converted into Joule heat. It is considered that melting occurs when the melting point or the glass transition temperature of the organic compound is low. Further, electric field is concentrated on the portion where a pin hole or a scratch exists, whereby the dielectric breakdown occurs. As to the factors (4) and (5), the deterioration progresses even if the phosphor comprised of the organic compound or organic light emitting element is preserved at a room temperature. The factor (4) is known as a dark spot, and arises from oxidization and reaction with moisture of a cathode. As to the factor (5), the organic compound used in the organic light emitting element is an amorphous material, and it is considered that almost no amorphous material exists in which the amorphous structure is kept stable since amorphous materials are crystallized by the long preservation, change through the elapse of time and heat generation.
The dark spot has been considerably suppressed because of the improvement of a sealing technique. However, the actual deterioration is occurred with the combination of the above factors, and thus, it is difficult to commonly understand the actual deterioration. The typical sealing technique is known as a method of making an organic light emitting element formed on a substrate airtight by a sealing member and providing a drying agent in the space. However, it is considered that the phenomenon, in which not only the current flowing through the organic light emitting element but also the emission luminance are lowered when a constant voltage is continuously applied, originates in the property of the organic compound.
A low molecular weight organic compound and a polymer organic compound are both known as an organic compound for forming an organic light emitting element. As one example of the low molecular weight organic compound, copper phthalocyanine (CuPc), or á-NPD (4,4′-bis-[N-(naphthyl)-N-phenyl-amino]biphenyl) or MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine), which is an aromatic amine-based material is known as the hole injecting layer, and tris-8-quinolinolate-aluminium complex (Alq3) or the like is known as the light emitting layer. As the polymer organic light emitting material, polyaniline, polythiophene derivative (PEDOT) or the like is known.
It is considered that the low molecular weight organic compound formed by an evaporation method has remarkable variety in comparison with the polymer organic material from the viewpoint of the variety of materials. However, in any case, the organic compound constituted of only a basic structural unit is rare. There may be the cases where different kinds of the organic compounds are combined, an impurity is mixed into the organic compound in a manufacturing process, and various additives such as a pigment are added to the organic compound. Further, among these materials, a material deteriorated due to moisture, a material easily oxidized and the like are included. Moisture and oxygen can be easily mixed from an atmosphere. Thus, care needs to be taken in handling the materials.
It is known that chemical bond is changed into double bond and the structure containing oxygen (—OH, —OOH, >C═O, —COOH and the like) when the organic compound is subjected to light deterioration. Therefore, in the case where the organic compound is provided in the atmosphere containing oxygen, or in the case where oxygen or H2O as an impurity is included in the organic compound, it is considered that the bond state changes, and the deterioration is promoted.
In the field of a semiconductor technique, in a semiconductor element having semiconductor junction, such as a diode, an impurity that arises from oxygen forms a local level in a forbidden band, which is a cause of junction leakage reduction and lifetime of or carrier. Thus, it is known that the impurity remarkably reduces the characteristics of the semiconductor element.
Oxygen molecules are peculiar molecules in a base state and also in a triplet state since a highest occupied molecular orbital (HOMO) is in condensation polymerization. Generally, the excitation process from triplet to singlet is forbidden transition (spin forbidden), and thus, is hard to occur. Therefore, oxygen molecules in the singlet state are not generated. However, when the molecules in the triplet excitation state (3M*) that is a state with higher energy than that of the singlet state exist around the oxygen molecules, energy transfer such as the following occurs. Thus, the reaction in which the oxygen molecules in the singlet state are generated can be found.3M*+3O2→1M+O2  Formula 1
It is said that 75% of the excitation state of molecules in a light emitting layer of an organic light emitting element corresponds to the triplet state. Therefore, in the case where oxygen molecules are mixed in the organic light emitting element, the oxygen molecules in the singlet state can be generated by the energy transfer in the formula 1. The oxygen molecules in the singlet excitation state have ion properties (there is polarization in electric charge). Thus, it is considered there is a possibility that the oxygen molecules react with the charge polarization generated in the organic compound.
For example, since a methyl group is electron donor in basocuproin (hereinafter referred to as BCP), carbon directly bonded to a conjugate ring is electrified in positive. As shown in the following chemical formula 1, singlet oxygen having ion properties reacts with oxygen molecules in positive electrification if the oxygen molecules exist. Thus, there is a possibility that carboxylic acid and hydrogen are generated as shown in the following chemical formula 2. As a result, it is expected that the electron transporting property is lowered.

The present inventor has found that an impurity such as oxygen or H2O contained in an organic compound causes various types of deterioration such as reduction of luminance in an organic light emitting element and an organic light emitting device using the same based on the study described above.
In the organic light emitting element having the layer comprised of the organic compound between a cathode and an anode, and in the light emitting device structured using the organic light emitting element, it is necessary to reduce oxygen concentration that brings about the reduction of luminance and deterioration of an electrode material, such as a dark spot.
A preferred applied example using the organic light emitting element is an active matrix drive light emitting device, in which a pixel portion is formed in the organic light emitting element. A thin film transistor (hereinafter referred to as TFT) as an active element is provided in each pixel. However, it is known that characteristic values such as threshold voltage fluctuate due to contamination of an alkali metal with respect to the TFT formed using a semiconductor film. In the present invention, an appropriate structure for forming the pixel portion by combining the organic light emitting element, in which an alkali metal with a small working function is used in a cathode, and the TFT is required.
The active matrix drive light emitting device in which the pixel portion is formed by combining the organic light emitting element and the TFT is structured by appropriately combining a semiconductor material containing silicon as its main constituent and an inorganic or organic insulating material containing silicon as its constituent. The external quantum efficiency of the organic light emitting element does not still reach 50%. Thus, most of injected carriers are converted into heat, whereby the light emitting element is heated. As a result, thermal stress is applied to the light emitting element and acts on the respective layers forming a pixel. There occurs a defect that a crack is generated if the thermal stress is large.